nat-traverse

nat-traverse is a software that establishes connections between nodes which are behind NAT gateways, i.e. hosts which do not have public IP addresses.

Additionally, you can setup a small VPN by using pppd on top of nat-traverse. nat-traverse does not need an external server on the Internet, and it isn't necessary to reconfigure the involved NAT gateways, either. nat-traverse works out-of-the-box.

Limitations:

• Only IPv4 is supported, nat-traverse won't work with IPv6 addresses. Even though it would be relatively trivial to add IPv6 support, I refrained from doing that, as there's no need to use NAT with IPv6 (the address space IPv6 provides is sufficient).